Try evmap for read-only requests

[Thomasdezeeuw]

For most use cases the majority of the request will be read-only (i.e. GET), having to go through the db::actor will likely (TODO: confirm this) become a bottleneck. evmap could help with improving performance here, removing the bottleneck on the db::actor.

Steps:

1. Get a baseline performance number of read-only requests. Could be done with YCSB workload C.
2. Update to use evmap.
3. Get a new number to compare to step 1.